Who is Johanna from Effi Briest and what is their importance?

Johanna is Geert von Innstetten's maid. She was born in Pasewalk, and she is described as plump, pretty, and fair-haired, though "no longer entirely youthful". Johanna sits with Effie when the Chinaman's ghost frightens her, and she does her best not to reveal the secrets. She enjoys reading fashion magazines, and then she grumbles about not being able to afford nice things. Johanna is the only servant to accompany the Innstettens to Berlin.